a replacement would be a gt240, ddr5 version. it would give similar performance. anything better than that is an upgrade. a 5770 or gts450 would be a decent upgrade. A gts250 may be a cheaper option, gives similar performance to a gts450 but lacks dx11 features.
If I were you, I'd go with an upgrade. If you buy a card that's already old, you are shooting yourself in the foot if you wanna play a new game. I would get a GTX 460 or GTX 560Ti. These are from the lower end of Nvidia's GPU lineup, but they are nonetheless incredible cards. Also, before asking questions like these always check here first, and find the best card for your budget. Hope I helped at least a little!